A new method to simultaneously invert for Q structure and source parameters was used on a set of 635 microearthquakes (0.9<M<2.0) in the Kaoiki area of southern Hawaii. Approximately 2800 signals were analyzed which had been recorded by 6 short period vertical seismographs at epicentral distances of a few to 10 km. The hypocentral depths ranged from 0 to 14 km, with the bulk of the sources in the 7.5--10.5 km range. The hypothesis to be tested was that the source volume of the M=6.6 Kaoiki mainshock of November 16, 1983, may be heterogeneous in attenuation distribution. We assumed that the observed P wave displacement spectra could be modeled by a source spectrum with an ω-2 high-frequency decay, a single-layer resonance filter to account for local site resonances and whole path attenuation along the ray path. In a next step the attenuation factor Q was constrained by tomographically reconstructing the three-dimensional Q structure for the source region and using it as starting model for a nonlinear inversion of the corner frequency, the seismic moment M0, and a new Q value. This process was iterated until the results changed less than 0.1% and were accepted as final. The average Q was approximately constant and very low (105<Q<115) between 0 and 7 km depth. It increased to approximately 160 between 9 and 11 km. In the depth ranges where the spatial coverage was good (3--9 km) the Q values in the NW part of the 1983 aftershock volume were larger by approximately 10--15% compared to those in the SE part. This 12% contrast correlates with other evidence for heterogeneity. The NW region shows high coda-Q, no surface faulting, medium seismicity rate, and medium precursory quiescence, while the SE region shows low coda-Q, pervasive faulting, high seismicity rate, strong precursory quiescence, and possibly low b values. We conclude that the Kaoiki source volume is strongly heterogeneous with the contrasting density of cracks controlling the difference in crustal parameters.

The new simultaneous inversion technique is well suited to derive detailed Q structure and source parameter information from microearth spectra recorded at close epicentral distances. ¿ American Geophysical Union 1990